Feel free to pick and choose the option you find best suits the following defintions of terrorism, widely accepted by most intellectuals and used in British and American law: 1. the use or threat is designed to influence the government or an international governmental organisation or to intimidate the public or a section of the public. 2. the use of violence and threats to intimidate or coerce, esp. for political purposes 3 the use or threat is made for the purpose of advancing a political, religious or ideological cause. 4. Government or resistance to government by means of terror. By biggest, we obviously refer to its global reach and impact. Civilian deaths, human rights abuses, you get the picture. EDIT: There are too many bodies to name, Thrown one in if i've missed a glaring one.